Rotary cutting of a ceramic tile presents various difficulties because of the hardness and smoothness of both the glazed protective layer of the tile and the tile substrate material itself. It is known to cut ceramic tiles by initially using a small diameter drill bit to cut a small hole in the protective layer to minimise the possibility of cracking the protective layer and the tile substrate. Next drill bits of successively larger diameters are used to enlarge the hole to the required size once the tile has been pierced with a small hole and the risk of damage to the tile is reduced. However, this known method suffers from the drawback that several drill bits of different fixed diameters are required in order to cut a hole in the tile to a desired diameter.
One known drill bit which seeks to overcome the above mentioned problem is disclosed by International patent publication no. WO 03/061927A1. This drill bit has a cutting plate seated in one end of a cylindrical shank with a rotational axis. Viewed in side elevation, the cutting plate has a rectangular main body portion seated in the shank and a triangular portion extending from the main body portion. The triangular portion is terminated by a small pyramidal portion with a small chisel edge at its foremost end. The chisel edge is defined by a pair of inclined front faces and a pair of inclined side flanks. The intersection between the pair of inclined side flanks forms the chisel edge. The inclined front faces define the length of the chisel edge. Each inclined side flank is inclined axially rearward away from the chisel edge until each meets a respective inclined side face. Each inclined side face is also inclined axially rearward away, but at a steeper angle than that of an adjacent inclined side flank. Each inclined front face is inclined axially rearward from a respective end of the chisel edge until each meets a respective axis parallel front face. In use, the hole in the ceramic tile is steadily increased from a very small diameter to the full working diameter of the drill bit, in one operation.
A primary cutting edge is formed at the intersection between the inclined side flanks and a respective rotationally leading inclined front face. A second cutting edge is formed at the intersection between the inclined side faces and a respective rotationally leading front face. The inclined side flanks and side faces are also inclined radially inward from their respective rotationally leading primary and secondary cutting edges. However, the inclined front faces are inclined radially inward from their respective rotationally trailing primary cutting edges, whilst the parallel front faces are not radially inclined, one way or the other. For the sake of clarity, a face or edge that is ‘inclined radially inward’ from a given point is one that departs from said point in a direction tending, or inclined, towards the axis of rotation.
Thus, a cross section through a plane normal to the rotational axis and including the primary cutting edges would reveal that the inclined side flanks and front faces circumscribe a first parallelogram shape. In this first parallelogram the primary cutting edges are located in the corners furthest apart and it is the inclined side flanks that provide relief to the primary cutting edges.
Likewise, another cross section through any plane normal to the rotational axis, including the secondary cutting edges, would reveal that the inclined side faces and front faces circumscribe a second parallelogram shape. In this second parallelogram the secondary cutting edges are located in the corners furthest apart and the inclined side faces provide relief to the secondary cutting edges.
In both parallelograms, opposing faces are parallel. In the case of the second parallelogram, the included angle at the secondary cutting edges is 90° minus the angle of inward radial inclination of the inclined side faces. This is because the parallel front faces are not radially inclined. In the case of the second parallelogram, each inclined side flank and front face is inclined radially inward from a respective intersecting primary cutting edge. Therefore, the included angle at the primary cutting edges would be 90° minus the sum of the angles of inward radial inclination of the inclined side flanks and side faces. Accordingly, the first parallelogram is more collapsed and has sharper primary cutting edges than the second parallelogram which is fuller and has duller, or less sharp, secondary cutting edges. For the sake of clarity, a more ‘collapsed’ parallelogram is intended to mean one that circumscribes a smaller area than a fuller, or less ‘collapsed’, parallelogram having equivalent sides. Of course, a rectangle circumscribes the fullest possible area of a parallelogram of a given length sides.
Whilst the sharp primary cutting edges of the drill bit disclosed by WO 03/061927 A1 have an initial advantage of cutting a ceramic tile more quickly this initial advantage is short lived and these sharp primary cutting edges soon wear and become blunt when used upon relatively hard and abrasive materials such as ceramic tiles. Such sharp cutting edges are also brittle and more prone to chipping. This is especially problematic at the foremost working end of the drill bit in the region of the chisel edge where the cutting plate's geometry is small and there is not enough spare material to re-sharpen chipped cutting edges more than a few times, if at all. This reduces the life span of the drill bit. Further, if the inclined side faces are steeply inclined axially rearward with an included angle falling within the range of 30° to 60° then the cutting plate's geometry in the region of the chisel edge becomes especially small. In this case breakage of the cutting edges could result complete removal of the foremost working end of the cutting plate making its re-sharpening impossible.